n. A situation in which a CMOS device conducts power to ground because its safe input voltage levels were exceeded, usually resulting in the device’s failure. SCR latchup happens when the input to a gate or other CMOS device goes above the part’s Vcc or below ground. This might seem unlikely in the benign environment of a digital circuit, but fast edges sent over even a few inches of PCB track can generate overshoot or undershoot from impedance mismatches. Sometimes there’s enough over- or undershoot to drive the part into latchup. It’s also a problem when plugging an unpowered board into a hot socket; if the input pins get signals before power and ground make a connection, SCR latchup can result.
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